Ball screw drives are used in electromechanical power steering systems of motor vehicles. Such power steering systems have, on the one hand, racks and pinions or, on the other hand, belt drives for force transmission. In belt systems, the assisting motor is connected to the ball screw drive by means of a belt. The ball screw drive, consisting of a ball nut and threaded spindle, converts the rotary movement of the ball nut into a linear movement of the spindle, with the result that the rack is displaced in the axial direction to steer the wheels. The belt acts on the ball nut via a belt pulley connected to the ball nut for rotation therewith. Here, there is the risk of a drop in belt tension if, in the case of elastically supported ball screw drives, the ball nut tilts about its axial axis. Here, the belt is at least partially decoupled from the belt pulley or the motor, with the result that the assist force applied by the motor for steering is reduced. Moreover, the nonuniform stressing of the belt leads to an undesired reduction in its service life.
Particularly in the so-called curb pressure test, the ball screw drive is exposed to high loads. The curb pressure test is carried out for production vehicles to determine the rack diameter of the steering system.
DE 10 2010 003 233 A1 discloses a ball screw drive of a motor vehicle steering system with a ball nut which is mounted in a frame by means of a rolling bearing, wherein an elastomer is provided between the rolling bearing and the frame, which elastomer supports the rolling bearing in a prestressed manner with respect to the frame and prevents sound conduction from the interior of the steering gear via the rolling bearing to the outside.
DE 10 2010 029 767 A1 discloses a ball screw drive for motor vehicle steering systems, in which the airborne-sound radiation is reduced to a minimum. For this purpose, the ball nut is mounted in a housing via a bearing, wherein the bearing has axial and radial damping devices which are arranged between the bearing and bearing seat.
The aforementioned ball screw drive bearing arrangements can partially compensate bearing-side bending moments and axial forces acting on the nut. However, a constant belt tension cannot be achieved with these embodiments.
DE 10 2011 117 723 A1 discloses a ball screw drive of a motor vehicle steering system, in which the ball nut is movable relative to the belt pulley in the axial and/or radial direction. Although the belt pulley is decoupled radially and/or axially to a certain degree from the ball nut, said decoupling is not sufficient to compensate the axial forces arising through special events, such as the curb pressure test.